1. Field of the Invention
This invention relates to an expansion temperature tolerant, dry, initially non-tacky, expandable sealant and acoustic baffle component for sealing an automobile body cavity and which is capable of undergoing expansion and becoming tacky at a temperature experienced by the automobile body cavity during oven curing of primers, sealers, and top coat applied to the vehicle. It also concerns an improved composition for forming an expandable sealant and acoustic baffle component which is capable of undergoing a desired degree of full expansion over a wider temperature range than has heretofore been the case. The invention in certain embodiments involves a sealant and acoustic baffle component of desired characteristics which also has improved structural integrity to provide added reinforcement for the cavity-defining structure of the vehicle body. The invention also relates to a novel method for preparing an expandable sealant and acoustic baffle component composition on a continuous processing basis, thus reducing the variability often observed with "batch type" processing.
2. Discussion of the Prior Art
Automotive vehicle bodies have a variety of hollow posts, cavities, passages and similar areas which can transmit undesirable noises, air currents, moisture, dust and other airborne particulates unless sealed to prevent infiltration of such sounds or contaminants. U.S. Pat. No. 5,373,027, issued to the assignee hereof, describes a dry, initially non-tacky, heat expandable sealant and baffle component which may be formed into a desired shape so that upon insertion into post, cavity or passage of the vehicle body, the component will undergo heat-induced expansion when the vehicle body is conveyed through a baking oven forming a part of the primer or paint curing step of the automobile manufacturing process.
In accordance with the '027 patent, the composition for preparing the dry, expandable sealant and baffle product includes a major amount of an ethylene-.alpha., .beta. ethylenically unsaturated carboxylic acid copolymer which has been partially neutralized with a metallic ion such as zinc, a minor amount of a blowing agent such as a modified azodicarbonamide and a minor amount of a low molecular weight resin tacifier which has the property of imparting tackiness to the outer surface of the component when the latter is raised to an elevated temperature and thereby expanded by the blowing agent.
The constituents of the composition for preparation of the dry, expandable sealant and baffle component are functional to form an expanded shape within the cavity of the automobile body when it is conveyed through a conventional paint or primer bake oven. Conventional ovens for baking or curing a finish applied to an automobile body typically operated at temperatures in the range of about 140.degree. C. to about 200.degree. C., depending upon the coating to be dried and cured. The automobile body usually transits a particular baking oven in a time period of from about 10 to 15 minutes to as much as 2 hours. To that end, prior expandable baffle compositions have been formulated to preferably undergo expansion at a temperature of about 135.degree. C. to about 185.degree. C. The dry, expandable sealant composition and product of the '027 patent has enjoyed substantial commercial success throughout the automotive manufacturing field. Recent popularity of expanded-in-place sealant and baffle products for automotive applications has been accelerated by the ease of use of the baffle elements and the fact that each can be appropriately sized and configured to custom fit specific body cavity areas. Furthermore, the fact that baffle expansion can be accomplished in situ while an automobile is passing through a baking oven avoids the necessity of providing additional equipment and manufacturing operations for that specific purpose.
Not unexpectedly, the focus of the paint and/or primer baking process in automotive manufacturing procedures is to properly and efficiently cure the paint and/or primer coating that has been applied to the surface of the vehicle body. Processing time is of the essence in the manufacture of automobiles and similar vehicles, and therefore the residence time of a vehicle body in the paint and/or primer baking oven is carefully controlled to effect the required curing of the paint and/or the primer, in a minimum time. It is to be appreciated that because of the many different cavities and passages in an automobile body, both as to location, length, and relative distance from the surface of the body which is being treated with heat to bake the paint or primer, the actual temperature experienced in particular cavities and passages of the body during the paint and/or primer curing operation can vary significantly, with certain cavities and passages being either raised to substantially higher temperatures than other cavities or passages during the baking cycle, or not elevated to an adequate temperature to insure full expansion of an expandable baffle element.
Engineers select the most advantageous vehicle body cavity locations for expandable baffle elements based on the susceptibility of particular cavities to transmit undesirable sounds or contaminants. That baffle location selection normally does not take into account the differences in the degree of heating of the location during the baking or curing cycle. For example, in high metal mass areas, heat energy dissipates more rapidly than in low metal mass areas. Thus, the expandable baffle material does not reach a sufficiently high temperature level to effect required full expansion of the baffle product prior to exit of the automobile body from the baking and curing oven. On the other hand, other baffle product locations may result in overheating of the baffle material because of its proximity to the direct heat source, the nature of the cavity defining metal surrounding the expandable baffle material, or the remoteness of the cavity from the outside surface of the vehicle body. Upright pillars can exhibit a chimney effect that has a bearing on the temperature realized in the that body cavity zone.
Thus, formulation of a suitable dry expandable sealant and baffle product requires that a number of diverse and functionally disparate parameters and conditions be satisfied. The composition must be made up of economical constituents that can be processed in an efficient manner preferably using existing equipment. The composition must be readily formable into a self-sustaining body of predetermined size and shape. The body should be capable of undergoing a of expansion from its original size, and to expand substantially uniformly in all directions unless constrained from doing so by a carrier for the body, or by a suitable baffle structure associated with the body, in order to effectively seal the cavity. The body formed from the expandable composition must be essentially dry and non-tacky as formed for insertion in a specific cavity of the automobile body.
Most particularly, given the fact that different cavities in the vehicle body are raised to temperatures which differ from one cavity to another, the composition used to prepare the expandable sealant and baffle product must be capable of undergoing a high degree of expansion at even the lowest temperature likely to be experienced in a cavity in which an expandable part is positioned. Similarly, the expandable sealant and baffle product must not undergo carbonization and/or burning at the highest temperature that is likely to be experienced in one of the expandable part-receiving cavities during the vehicle paint and/or primer curing cycle.
In addition, the composition must be of a nature such that when formed into a expandable part for insertion in a vehicle body cavity to be cured in a paint or primer baking oven, the part has physical network-like characteristics which enhance entrapment of the gasses which are produced by the blowing agent which is incorporated into the formulation, thus assuring that the part undergoes a high degree of retained expansion in use.
It is also desirable that certain embodiments of the expandable sealant and baffle product have the functional property of providing additional structural reinforcement for the cavity-defining structure of the vehicle body, while retaining its noise, air and moisture barrier characteristics.